Instantaneous and Selective Bare Eye Detection of Inorganic Fluoride Ion by Coumarin–Pyrazole‐Based Receptors

A series of rationally designed coumarin–pyrazole‐based scaffolds, equipped with N―H and C―H hydrogen bond donors ( R1–R5 ) and containing various electron‐withdrawing groups at key positions, are synthesized and characterized in order to investigate their inorganic fluoride binding properties in highly competitive media (1:1 DMSO–water). Only one, 3‐{4‐[(2,4‐dinitrophenyl)‐hydrazonomethyl]‐1‐phenyl‐1H‐pyrazol‐3‐yl}‐chromen‐2‐one ( R1 ), of the five compounds synthesized, is found to be capable to selectively detect inorganic fluoride via naked eye amongst other anionic species in aqueous media. Qualitative and spectroscopic studies exhibit that receptor R1 has the potential of showing instantaneous change of color from yellow to pink upon addition of sodium fluoride (0.95 ppm) in aqueous media, at concentration lower than that recommended by World Health Organization (1 ppm). Intensity of color increases with increasing fluoride concentration till 5 ppm, beyond which intensity of color change becomes saturated. This has established the applicability of this receptor for assessment of the level of fluoride in water. Anion binding studies carried out by UV–visible titration portrayed substantial bathochromic peak shift from 410 to 495 nm, upon addition of varying concentrations of aqueous sodium fluoride solution, which has validated the color change. Jobs plot data confirmed 1:1 stoichiometry between R1 and fluoride ion. ¹H‐NMR investigation reveals that the deprotonation of N―H hydrogen donor group of receptor R1 and its interaction with fluoride ion is responsible for the observed color change.     

基于含偶氮酚羟基席夫碱新型简单比色传感器

A novel N-(2-hydroxy-5-chlorodibenzophenone)-N0-[2-hydroxy-5-azophenyl-benzaldehyde]-1,2-diaminobenzene receptor has been synthesized by simple steps with good yields. The anion recognition properties were studied by ultraviolet-visible spectroscopy. The resultsshowed that the receptor had a higher affinity to F^-, AcO^-, and H₂PO₄^-, but no evident binding with Cl^-, Br^-, and I^-. Upon addition of the three former anions to the receptors in DMSO, the solution exhibited an obvious color change from colorless to yellow, which could be observed by the naked eye, thus the receptor could act as a fluoride ion sensor even in the presence of other halide ions. The UV-Vis data indicates that a 1:1 stoichiometric complex is formed through hydrogen bonding interactions between receptor and anions.     

A novel, simple, colorimetric receptor based on 2′,4′-dinitrophenylhydrazone for acetate ion in organic medium

A novel 1,3-di(2′,4′-dinitrophenylhydrazone)-5-nitrobenzene receptor has been synthesized by simple steps with good yields. The anion recognition properties were studied by ultraviolet-visible (UV-Vis) spectroscopy. The results showed that the receptor had a higher affinity to F^?, CH₃COO^? and H₂PO ₄ ^? , but no evident binding with Cl^?, Br^?, and I^?. Upon addition of the three former anions to the receptors in dimethyl sulphoxide (DMSO) at 298.2 ± 0.1 K, the solution exhibited an obvious color change from yellow to mauve that could be observed by the naked eye, thus the receptor could act as a fluoride ion sensor even in the presence of other halide ions. The UV-Vis data indicates that a 1:1 stoichiometry complex formed through hydrogen-bonding interactions between receptor and anions. The hydrogen bond between phenylhydrazone –NH and acetate or fluoride anion was determined on the basis of ¹H nuclear magnetic resonance (NMR) experiments.     

A chloride selective sensor based on a calix[4]arene possessing a urea moiety

New calix[4]arene derivative 1 of 1,3-alternate conformation with a ureido moiety has been synthesized in high yield and examined for its anion recognition abilities towards anions such as fluoride, chloride, bromide, iodide, nitrate and acetate by ¹H NMR and UV-vis spectroscopy. The results show that receptor has strong binding affinity for chloride ions. A chloride ion selective electrode (ISE) was also formed which showed excellent selectivity over all the other anions tested. The limit of detection is 2.51 × 10⁻⁵ mol dm⁻³.     

A novel anthracene-based receptor: Highly sensitive fluorescent and colorimetric receptor for fluoride

A new highly sensitive colorimetric receptor 1 for fluoride based on anthracene-9,10-dicarbaldehyde bis-p-nitrophenylhydrazone was designed, synthesized and characterized. Experiments showed that the receptor 1 can selectively recognize the fluoride in DMSO and even in 95/5 DMSO/H₂O (v/v) mixtures. The ability of recognition and the bond between receptor 1 and anions were determined using visual inspection, UV-vis and fluorescence analyses. In addition, ¹H NMR experiments were carried out to explore the nature of interaction between receptor 1 and fluoride. Finally, analytical application and detection of fluoride in toothpaste have been studied.     

A tripodal receptor bearing catechol groups for the chromogenic sensing of F ions via frozen proton transfer

A neutral tripodal Schiff base receptor (3) having catechol as end groups has been synthesized and characterized with the help of spectroscopic and single crystal X-ray crystallographic studies. The receptor behaves as a visually detectable optical sensor for F⁻ ions in DMSO, sensitive enough to recognize the ions up to a concentration 1×10⁻⁵ M with naked eye. The chromogenic response is based upon the deprotonation of the highly acidic catechol moieties in the presence of highly basic F⁻ ions in a polar solvent like DMSO.     

A naphthyridine-based receptor for sensing citric acid

A naphthyridine-based charge neutral receptor has been designed and synthesized. Its complexation with a series of carboxylic acids involved in the Krebs cycle has been studied by ¹H NMR, UV-vis and fluorescence methods. The receptor shows strong binding to citric acid (K_a = 1.60 × 10⁵ M⁻¹) and is also able to distinguish diastereomeric maleic acid from fumaric acid by fluorescence.     

Al selective an efficient colorimetric receptor derived from 5-aminouracil

An interference-free naked-eye recognition of Al³⁺ at its micromolar level has been done in 5% aqueous DMSO solution employing a Schiff base 5-[(2-hydroxy-5-nitro-benzylidene)-amino]-1H-pyrimidine-2,4-dione (receptor 1) which is an intramolecular charge transfer (ICT) probe. The pyrimidine and nitrophenyl groups serve as electron rich (donor) and deficient (acceptor) pockets in receptor 1 exhibiting a broad ICT band at 434 nm (olive green). The concomitant additions of Al³⁺ as its chloride salt to the 5 × 10⁻⁵ M aqueous DMSO solution of the receptor 1 lead hypsochromic shifting of its ICT band to 395 nm (colorless). The same ICT band undergoes a marginal bathochromic shifting (6 nm) along with a hyperchromic shift on separate additions of a basic anion like F⁻, CH₃COO⁻ and H₂PO₄⁻ to the receptor 1 and faced almost similar fate on concomitant additions of Al³⁺ as mentioned above.     

Colorimetric recognition of anions using preorganized tetra-amidourea derived calix[4]arene sensors

The synthesis and the spectroscopic studies of the amidourea based calix[4]arene sensors 1 and 2 are described. The 4-nitrophenyl based sensor 1 was synthesized in two steps from the corresponding calix [4]arene tetraethyl ester and shown to give rise to color changes in the UV-vis spectra in DMSO upon recognition of pyrophosphate and fluoride. Fitting the changes in the absorption spectra using nonlinear regression analysis indicated strong binding of several anions by 1 such as acetate and hydrogen phosphate in 1:1 (Host:Guest) stoichiometry, and at higher concentration in 1:2 stoichiometry. The preorganized calix-cavity was, however, not found to host chlorine while binding of bromide was determined. At high concentrations of these anions, significant colorimetric changes were also observed that were clearly visible to the naked eye for both pyrophosphate and fluoride. The phenyl analogue 2 was made to enable analysis of the anion recognition using 1H NMR titrations and showed that ions such as phosphate were bound in 1:1 stoichiometry, whereas the "urea" protons were shown to be significantly affected upon coordination to the anion.     

Hexaphyrin(1.0.1.0.0.0). A new colorimetric actinide sensor

Hexaphyrin(1.0.1.0.0.0) (isoamethyrin) undergoes a significant color change in the presence of UO₂²⁺, PuO₂²⁺, and NpO₂²⁺. The complexation of the first of these dioxo actinide cations was studied in semi-quantitative fashion in 1:1 MeOH-CH₂Cl₂. Under these conditions, the detection limit for UO₂²⁺ was found to be ca. 5.8 ppm by naked eye monitoring and <28 ppb by UV-vis spectroscopy. Isoamethyrin does not undergo a color change in the presence of most transition metals or when exposed to Gd(III). Isoamethyrin thus constitutes an attractive alternative to 2,2′(1,8-dihydroxy-3,6-disulfonaphthylene-2,7-bisazo)-bisbenzenarsonic acid (AzIII) and 2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenol (BrPADAP), systems currently used as actinide cation sensors.     

Colorimetric detection of potassium ions using aptamer-functionalized gold nanoparticles

Herein, a simple and novel colorimetric method for detection of potassium ions (K⁺) was developed. The colorimetric experiments revealed that upon the addition of K⁺, the conformation of anti-K⁺ aptamer in solution changed from random coil structure to compact rigid G-quadruplex one. This compact rigid G-quadruplex structure could not protect AuNPs against K⁺-induced aggregation, and thus the visible color change from wine-red to blue-purple could be observed by the naked eye. The linear range of the colorimetric aptasensor covered a large variation of K⁺ concentration from 5 nM to 1 μM and the detection limit of 5 nM was obtained. Moreover, this assay was able to detect K⁺ with high selectivity and had great potential applications.     

A high selective anion colorimetric sensor based on salicylaldehyde for fluoride in aqueous media

A new and simple salicylaldehyde-based sensor 1 designed for fluoride sensing has been investigated in DMSO and even in the 9/1 DMSO/H₂O (v/v) mixtures. The affinity constants of receptor 1 for anionic species in the 9/1 DMSO/H₂O (v/v) reveal that it is sensitive to F. Also, the color changes induced by anions can provide a way of detection by ‘naked-eye’. These result can be substantiated by the spectrum changes upon the addition of 25 equiv. anions to 1 in the 9/1 DMSO/H₂O solution. The further insights to the nature of interactions between the sensor 1 and F⁻ were investigated by ¹H NMR titration experiments in 9/1 DMSO-d₆/H₂O (v/v). In addition, the proposed binding mode between 1 and F⁻ was suggested.     

A simple naphthalene-based colorimetric sensor selective for acetate

A new naphthalene based receptor (L) has been designed and synthesized which shows a remarkable color change from colorless to pink on selective binding with acetate. The anion recognition property of the receptor via hydrogen bonding interactions is monitored by UV-vis, fluorescence, and ¹H NMR titrations. It is observed that in each case, the receptor shows a specific selectivity toward the acetate ion over other interfering anions. Thus, a significant bathochromic shift in UV-vis spectrum with a sharp pink color in ‘naked-eye’ makes the receptor suitable for the detection of the acetate ion.     

An efficient thiourea-based colorimetric chemosensor for naked-eye recognition of fluoride and acetate anions: UV-vis and HNMR studies

An efficient colorimetric chemosensor with a thiourea binding site and 2-amino-6-nitrobenothiazole as a signaling unit has been synthesized by dithiocarbamate approach. The chemosensor has been utilized for selective recognition of fluoride and acetate anions in dry DMSO solution by UV-vis and ¹H NMR titration experiments. The chemosensor has shown naked-eye sensitivity for both the anions in solution.     

A selective colorimetric chemosensor for lanthanide ions

The synthesis and complexing properties of a calix[4]arene derivate (6) carrying two spirobenzopyran moieties are described. The addition of lanthanide ions resulted in significant UV-vis spectral shifts (68-84 nm) in visible region. It indicates that the synthetic receptor can recognize lanthanide ions by naked eyes over other cations including Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe³⁺, Cu²⁺ and Zn²⁺. The mechanism of recognition was studied with ¹H NMR, UV-vis spectra and emission spectra. The receptor may be applied to sense lanthanide ions.     

A simple thiourea based colorimetric sensor for cyanide anion

A simple and easily synthesized colorimetric anion sensor, based on a thiourea moiety as a binding subunit on a 1,2-cyclohexane backbone and a p-nitrophenyl group as a signaling unit, has been synthesized in a one step procedure. The selective sensing of anions, particularly cyanide, has been investigated in DMSO by UV-vis titration, ¹H NMR titration techniques and through ‘naked eye’ observation experiments.     

Anion recognition by a novel Fipronil-based receptor: efficient deprotonation or stable intermolecular hydrogen bonding

Strong electron-deficient heterocycles of acetyl Fipronil (F3) was designed and synthesized, its ability for anion recognition was investigated by UV and NMR analyses. This novel Fipronil-based receptor F3 shows strong binding affinity with acetate (?10⁷ M⁻¹), phosphate or fluoride ion through efficient deprotonation. In addition, its interaction with chloride anion or other weak base anions through stable intermolecular H-bonding was also reported.     

Highly sensitive fluorescence "turn-on" indicator for fluoride anion with remarkable selectivity in organic and aqueous media

A simple, convenient, and inexpensive method has been developed to quantitatively determine fluoride anion concentration in acetonitrile as well as in water. The method exhibited a high selectivity and a great sensitivity toward fluoride anions through "turn-on" chromogenic and fluorogenic dual modes. The fluoride driven silyl deprotection and the subsequent spectral changes of hydroxyl coumarin were the operating foundations for the observed selectivity and sensitivity. (1)H NMR spectral titration with F(-) revealed that complete deprotection of a triisopropylsilyl (TIPS) group needed exactly 1 equiv of TBAF. UV-vis and fluorescence titration studies exhibited the appearance of a new intense absorption band centered at 434 nm and green emission peak at 500 nm, accompanied by bright yellow color development to the naked eye. An easy-to-prepare test paper, obtained by dipping the paper into the solution of TIPS-protected coumarin derivative, was able to detect F(-) in aqueous media. The method has also shown highly promising results in detecting all kinds of fluoride salts, regardless of being organic or inorganic, and thus could be potentially useful in real applications.     

An exclusive fluoride receptor: fluoride-induced proton transfer to a quinoline-based thiourea

A new quinoline-based tripodal thiourea has been synthesized, which exclusively binds fluoride anion in DMSO, showing no affinity for other anions including chloride, bromide, iodide, perchlorate, nitrate, and hydrogen sulfate. As investigated by ¹H NMR, the receptor forms both 1:1 and 1:2 complexes yielding binding constants of 2.32(3) (in log β₁) and 4.39(4) (in log β₂), respectively. The quinoline groups are protonated by fluoride-induced proton transfer from the solution to the host molecule. The 1:2 binding is due to the interactions of one fluoride with NH binding sites of urea sites and another fluoride with secondary ⁺NH binding sites within the tripodal pocket. The formation of both 1:1 and 1:2 complexes has been confirmed by theoretical calculations based on density functional theory (DFT).